The present invention relates to open digestion of liquid specimens which are to be thereafter analyzed by spectroscopy to determine metals content therein and, refers more particularly, to method and apparatus by which this open digestion is practiced.
Acid digestion is used, for example, to prepare surface water and ground water samples or specimens for analysis by spectroscopy to ascertain presence of various metals. This is done to effectively deal with recovery or safe disposal of the metals in the water. Metals of interest are numerous, cobalt, iron and lead, to name but a few, being examples.
The acid digestion generally will be carried out by placing the sample in a glass dish and heating the sample therein on a hot plate. A reflex cover is placed over the dish to condense vapors evolving in the digestion process and to prevent splatter of reagent acid added to the specimen as part of the digestion protocol.
In the heating of the specimen to reduce it from an initial volume of say 105 ml (100 ml water and 5 ml reagent) to a desired volume of, e.g., 5 or 10 or 25 ml, same is boiled slowly and the evaporation rate, due to the dish presenting a constant bottom surface heat transfer area, will be substantially constant. Due to constant evaporation rate, it becomes difficult to observe accurately reduction of specimen volume in the dish to a particular specimen level associated with a desired final specimen volume. As a result, specimens at volumes near that desired frequently pass to dryness before an operator realizes such is occurring, and the whole purpose of the protocol is vitiated.
Difficulty in marking a level at which a desired reduced volume is present can be appreciated by noting that in a large cross sectional area dish or beaker, a 1/8th inch depth of specimen may represent that volume. Noting a level change to 3/32nd or 1/16th inch can be hardly discernible but yet specimen volume of 25% to 50% below that desired can have taken place.
Utilization of hot plate heating of plural specimens in upright beakers at the same time has the additional disadvantages of possible misidentification of a given specimen with another on completion of heating, or chance of an operator accidentally knocking over a standing beaker.
It is desirable, therefore, that improved means and manner be provided by which liquid specimens can be treated in an open digestion/concentration operation without hazard that the specimens inadvertently will be dried, spilled or otherwise contaminated by events attending heretofore hot plate practiced protocols.